(1) Field of the Invention
The present invention relates to a solid-state imaging apparatus used for a digital camera and the like, and a manufacturing method thereof.
(2) Description of the Related Art
Along with the widespread use of an imaging device-related products (a digital camera, a camera-equipped mobile phone and the like), the market of the solid-state imaging apparatus has been remarkably developed. In the current of such development, the needs have changed to a wide angle in addition to a high sensitivity and a high pixel density (hereafter, called as “high pixel”) of the solid-state imaging apparatus due to a tendency to a thin camera module with a tendency to a thin digital still camera, a thin mobile phone and the like. A downsizing of a camera optical system indicates that a lens used for the camera module becomes to have a short focal length. More particularly, a light is incident on a solid-state imaging apparatus with a wide angle (in other words, a wide angle measured from a vertical axis of an incidence plane of a solid-state imaging device composing a solid-state imaging apparatus).
In the present, in a charged-coupled device (CCD) and a metal oxide semiconductor (MOS) Imaging device that are commonly used as solid-state imaging apparatuses, a solid-state imaging device (called as “pixel”) which are semiconductor integrated circuits having a light-receiving unit are arranged in a two-dimensional array, in which a light signal from an object is converted into an electric signal.
The sensitivity of the solid-state imaging apparatus is defined based on an amount of output current of a light-collecting device to an amount of incident light, so that leading the incident light surely into the light-collecting device is an important factor for improving the sensitivity.
FIG. 5 is a diagram showing an example of a basic structure of a conventional solid-state imaging device. As shown in FIG. 5, a light 58 (light indicated by dashed lines) which is incident vertically into a microlens 60 is separated into colors using one of red (R), green (G), and blue (B) color filters 2, and then converted into an electric signal at a light-receiving device 6. Since relatively high light-collecting efficiency can be obtained conventionally, the microlens is used in almost all solid-state imaging devices.
In the future development of a solid-state imaging device supporting a wide angle incidence, it is necessary to lead an incident light at a specific angle surely into the light-collecting device.
However, in the conventional microlens, the light-collecting efficiency decreases depending on the incident angle of a signal light. More particularly, in FIG. 5 the light 58 which is incident vertically into the microlens 60 can be collected with high efficiency, while a light 59 (light indicated by solid lines) which is incident obliquely into the microlens 60 is collected with relatively lower efficiency. This is because that the light 59 which is incident obliquely is intercepted at an Al interconnection 3 in a solid-state imaging device, so that the light 59 does not reach the light-receiving device 6.
As described above, the solid-state imaging apparatus is made up of multiple pixels that are arranged in a two-dimensional array. Therefore, in the case of incident light with a spread angle, the angle of incidence differs between the pixels in the central area and the pixels near the edge (refer to FIG. 2). As the result, there is a problem that the light-collecting efficiency of the pixels near the edge decreases than that of the pixels in the central area.
FIG. 3 is an example of a cross-sectional diagram showing pixels near the edge. The incident angle of the incident light is relatively greater at pixels near the edge. Therefore, the improvement of the light-collecting efficiency is sought by displacing electric wiring parts to the inner direction which means by shrinking.
FIG. 4 is a diagram showing a dependency on an incident angle of the light-collecting efficiency of the solid-state imaging device using a microlens. It shows that the light-collecting efficiency of the solid-state imaging device using a microlens is relatively high for the incident light of the incident angle of around 20°. However, the light-collecting efficiency declines suddenly for the incident light of the incident angle of more than 20°. As the result, the amount of light collected at pixels near the edge is about 40 percent of that at the pixels in the central area, and the sensitivity of the whole pixels is limited by the sensitivity of the pixels near the edge at present. This value further declines with the decrease of the pixel size so that its application to an optical system with a short focal length such as a small-sized camera becomes very difficult. Furthermore, in a manufacturing method thereof, there is a problem that further circuit shrinking is not possible.
In view of aforesaid problem, it is necessary to design a microlens which is able to support the incident angle in order to prevent a decrease of the sensitivity of the solid-state imaging device associated with the increase of incident angle. However, although the current pixel size of the solid-state imaging device is as extremely fine as 2.2 μm, finer size of pixel is needed in order to realize further high pixel density in the future. Thus, processing of microlens is executed on a sub-micron basis, so that forming the microlens by the currently used thermal reflow processing is not possible.
As described above, in order to realize the solid-state imaging device applicable to an optical system (an optical system with a high incident angle θ) with a short focal length for a thin camera, a new type of light-collecting device, which can be formed by easy fine processing and also the light-collecting device whose light-collecting efficiency is not lowered even when an incident angle is high, comparing with microlens, needs to be developed.
In recent years, along with development of a planar process technology represented by optical lithography and electron beam lithography, a light-collecting device (Subwavelength Lens: SWLL) having a periodic structure of a sub-wavelength draws an attention. Here, a sub-wavelength area indicates an area with the same size as the wavelength of a light concerned or an area smaller than that. A certain research group of a university has substantiated that an aspherical Fresnel lens is changed to an SWLL having grid pattern, so that a light-collecting effect can be expected (for example Non-patent reference 1). As a method, the conventional Fresnel lens (FIG. 1 (a)) is divided by an area 61 of λ/2n (λ: wavelength of incident light, n: refractive index of lens material), so as to obtain a linear approximation (FIG. 1 (b)) and a rectangular approximation (FIG. 1 (c)) in each area, and thus the SWLL is formed. In addition, in the same way, it has been reported that a line width in a structure in a sub-wavelength area can be controlled, so that a blazed binary optical diffraction device can be formed, and thus diffraction efficiency is improved (for example Patent Reference 1).
If the SWLL could be used as the light-collecting device for the solid-state imaging device, the microlens could be formed using a general semiconductor process, and also the shape of microlens could be controlled without limitation.
However, the divided period of the SWLL (for example the area 61 in FIG. 1) is strongly dependent on a wavelength of incident light. Therefore, the divided period becomes 0.1 to 0.3 μm in an optical wavelength area. By the aforesaid method, the blazed binary optical diffraction device needs to be formed by further microstructure (0.01 to 0.1 μm), and forming such microstructure is extremely difficult by the currently available process.
Non-patent Reference 1: Opt. Eng. 38 870-878, D. W. Prather, 1998
Patent Reference 1: Japanese Laid-Open Patent 2004-20957